The invention relates to a circuit for balancing capacitor voltages at capacitors in a DC circuit.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Electrical assemblies often have different voltages and voltage levels for the energy supply and the energy distribution to their electrical components. For example, for electric converters with a DC circuit or with a DC link it is often inherently necessary, via a temporary storage of electrical energy, reliably to provide or decrease the DC voltage depending on the operating mode and operating point in the DC circuit.
Capacitors are particularly suitable as intermediate energy stores for the often dynamic intermediate storage and discharging processes at the DC circuit, due to their known electric properties. Many uses of electrical converters require high DC voltages in the DC circuit, which can be up to 800V and more. Since the working voltages of the capacitors which are suitable and, from an economic standpoint, available are limited, often two or more capacitors must necessarily be connected in series in the DC circuit so that they can be operated with the DC voltage of the DC circuit. Capacitors have, in particular, the undesirable property for normal operation in the DC circuit, due to structural and material-related configurations and tolerances, of discharging themselves over a, mostly individual, time period. In this process, a “leakage current”, also known as a residual current, flows via a leakage resistance of the capacitor, where the charge on the capacitor and thus also the voltage across the capacitor decreases.
In this context, normal operation should be understood as an operating state of the electrical assembly, that is, particularly also of the electrical converter, where no switching off of the energy distribution or the energy supply for the DC circuit initiated by the user or the usage process is activated.
Since the leakage currents of the capacitors differ in general due to the aforementioned tolerances and therefore an unacceptably high capacitor voltage can arise at the capacitors connected in series in the DC circuit which have a lower or no leakage current, damage to or destruction of the capacitors is to be prevented.
A known approach to balancing the capacitors in a DC circuit involves the introduction of at least two electric resistors in a further series connection in the DC circuit, which is arranged parallel to the series connection of the capacitors. In each of these series connections, a center tap is arranged between two capacitors and two resistors, respectively electrically connected to one another. This simple form of balancing circuit now assumes the balancing of the capacitor voltages in normal operation. It is also suitable, however, following switching off of the electrical converter and thus interrupting the energy supply for the DC circuit, for discharging the capacitors as rapidly and reliably as possible. A decisive disadvantage of this solution is the fact that in the electrical resistors of the further series connection in the DC circuit, cross currents flow continually during normal operation, even if balancing of the capacitor voltages is not necessary. Thus, significant electrical losses are sometimes generated, which lessen the overall efficiency of the electrical converter.
EP 2 584 686 A1 discloses a circuit and a method for balancing capacitors connected in series in a DC circuit with a center tap between the capacitors, which is intended in particular to prevent the electrical losses of the balancing circuit described in the introduction. A further series connection is provided in the DC circuit and includes of two switch elements, which is connected in parallel to the series connection of capacitors and the center tap at the capacitors and a further center tap between the first and second switch element are connected via an inductor. On a voltage increase at the first or the second capacitor, a pulse frequency can be applied to the first and/or the second switch element and the first or the second capacitor is partially dischargeable during a respective pulse duration via the inductor and the respective switch element. Simply stated, via the inductor and the switch elements, the excess energy is “pushed back and forth” between the capacitors. A regulating unit herein determines the voltage via the capacitors. Depending on the voltage difference, the corresponding switch element is controlled via pulse width modulation (PWM) and the excess energy is displaced via the inductor.
This type of balancing, however, functions only for a capacitor bank with at least two series connections of capacitors in the DC circuit. This solution is also designed technically complex since for the PWM, at least one regulating unit with a processor is provided and the balancing circuit cannot be configured for the PWM without the processor.
FIG. 1 shows a schematic circuit diagram of a conventional balancing circuit for balancing capacitor voltages at capacitors C1, C2 in the DC circuit 2. The two capacitors C1, C2 are connected in series and arranged between a first potential DC+ and a second potential DC− of a DC voltage DC in the DC circuit 2. So that the capacitor voltages at the capacitors C1, C2 can fall off symmetrically, two balancing elements RS1, RS2 in the form of series-connected ohmic resistors are connected in parallel to the series-connected capacitors C1, C2. In each case, center taps 4, 6 introduced between the capacitors C1, C2 and the balancing elements RS1, RS2 are connected to one another via an electrical connection 9. Without this balancing of the capacitor voltages at the, with regard to type and electrical values, similarly selected capacitors C1, C2, due to material and design-related tolerances, an uneven voltage distribution would arise at the capacitors C1, C2. If, for example, a first capacitor C1 has a smaller leakage current than a second capacitor C2, the first capacitor C1 has a higher capacitor voltage applied to it which can lead, possibly, to the destruction or at least the damaging of the first capacitor C1. Typically, the values of the ohmic resistors introduced as balancing elements RS1, RS2 are equal and are selected such that they are able to conduct cross currents which amount to a multiple of the possible leakage currents of the capacitors C1, C2.
Such balancing circuits are often used in electric converters, in particular frequency converters with a DC circuit 2 or a DC link circuit where due to the size of the DC voltage in the DC circuit 2, a plurality of capacitors C1, C2 are connected in series. Typically, however, the maximum breakdown resistance of the capacitors C1, C2 configured, for example, as electrolytic capacitors and connected in series is insufficient for the DC voltage in the DC circuit 2, for which reason the capacitors C1, C2 are operated as described in series in the DC circuit 2. The balancing elements RS1, RS2 and their arrangement in the balancing circuit shown in FIG. 1 prevent an inadmissibly high voltage drop at the capacitors C1, C2, which can arise due to leakage currents, although significant power losses are to be accepted since the cross currents through the balancing elements RS1, RS2 also flow, when no balancing of the capacitor voltages is necessary
It would be desirable and advantageous to provide an improved circuit for balancing capacitor voltages at capacitors in a DC circuit to obviate prior art shortcomings and to enable the circuit to be self-controlling, while generating a low power loss and yet being efficiently both technically and economically.